National Energy Technology Laboratory,
Keywords: solid sorbents, CO2 capture, ab initio thermodynamics
Summary:CO2 is one of the major combustion products which once released into the air can contribute to global climate change. Solid sorbents have been reported to be promising candidates for CO2 sorbent applications due to their high CO2 absorption capacities at moderate working temperatures. Molecular modeling can play a role in identifying optimal sorbents and exploring the capture mechanisms. By combining thermodynamic database mining with first principles density functional theory and phonon lattice dynamics calculations, a theoretical screening methodology to identify the most promising CO2 sorbent candidates from the vast array of possible solid materials have been proposed and validated at National Energy Technology laboratory (NETL). The advantage of this method is that it identifies the thermodynamic properties of the CO2 capture reaction as a function of temperature and gas pressure without any experimental input beyond crystallographic structural information of the solid phases involved. The calculated thermodynamic properties of different classes of solid materials versus temperature and pressure changes were further used to evaluate the equilibrium properties for the CO2 adsorption/desorption cycles. The selected CO2 sorbent candidates are further considered for experimental validations. As we know from the principles of thermodynamics, at a given CO2 pressure, the turnover temperature (Tt) of an individual solid capture CO2 reaction is fixed and may be outside the operating temperature range (ΔTo) for a particularly capture technology. In order to shift such Tt of a solid into the range of ΔTo, its corresponding thermodynamic property must be changed by changing its structure through reacting (mixing) with other materials or doping with other elements. In this study, through investigating several mixed sorbent materials, we demonstrate that by mixing/doping different types of solids it’s possible for a sorbent to shift its Tt to the range of practical operating temperature conditions. In addition, this presentation also demonstrates that some solid sorbents exhibit dual-functionalities. For example, while some solids (e. g. Li2CuO2, Li5FeO4, Li2FeO2, Li2ZrO3, Na2ZrO3, etc.) serve as CO2 sorbents to capture CO2, these sorbents also serve as catalysts for converting CO (or NO) into CO2 (or NO2) and then capture these converted CO2 to purify the gas stream. Particularly, for pre-combustion, such sorbents can purify H2 with CO/CO2 impurities down to ppm level. As another example, some sorbents (e. g. CaFe2O4) can capture CO2 to form carbonate (CaCO3) and metal oxide (Fe2O3). During CO2 release cycle, both CaO & metal oxide (Fe2O3) serve as catalysts to convert CO2 into valuable chemicals.